Addressable heating element structures formed from resistive traces are used in a wide variety of applications including thermal ink jet (TIJ) printer heads, microelectronics, thermally assisted Magneto-resistive Random Access Memory (MRAM), actuation mechanisms for Microelectromechanical Systems (MEMS), operation of conglomerate pump systems and specialty devices such as that described in co-pending U.S. patent application Ser. No. 11/495,359, (which is hereby incorporated by reference in its entirety for all purposes) which may be used for drug delivery. The optimum heating profiles for each of these applications is different, requiring different designs.
Thin film heating structures are typically used in the microelectronic arena. Typically, multi-layer thin film heating structures are used for ink jet print heads, while thermally assisted MRAM may use either single or multi-layer thin film heating structures.
The multi-layer thin film heating elements used in thermal ink jet print heads are designed to reach an actuation temperature very quickly (within a few microseconds), maintain the actuation temperature for only a short period of time (a few microseconds), and then cool quickly. The objective is to heat rapidly in order to vaporize a substance, such as ink, and create a small gas bubble. The intention is to prevent heating more of the surrounding fluid than is necessary to generate the bubble and constrain the temperature increase to the area surrounding the bubble. As the bubble expands, some of the substance/ink is expelled from a holding chamber. Once the bubble collapses, capillary flow draws more of the substance/ink into the holding chamber from a reservoir. Once the ink is dispelled, the heater must be quickly cooled before the next expulsion, since simply holding the resistor at the high temperature does not expel more substance/ink. However, such rapid heating can have harmful cavitational effects to the surrounding materials, meaning that these heating systems are not necessarily effective or desirable for other applications.
Generally, single thin film heating elements are designed to heat either specific or indiscriminate areas for specific times to accomplish a predetermined objective. Often, when used for heating of target areas, existing TIJ or other heater structures will produce cross talk across adjacent target areas. This cross-talk will have the unintended consequences of heating the neighboring devices before actuation is desired. In applications such as MRAM or other arrayed devices unintended heating can have disastrous consequences for operation of the device. For example, if the structure is used in drug delivery applications, such as a microinjection device, unintended heating of adjacent wells could cause premature and inadvertent injection of the drug, possibly leading to adverse effects for the patient.
Moreover, as the area that is heated enlarges, whether or not such heating is intended, the power requirements increase. In battery operated devices, for example, unnecessary power consumption needlessly decreases the lifetime of the battery.
The ability to keep a desired area at a desired temperature while minimizing unwanted heating and thermal degradation is beneficial from the standpoint of operational efficiency, longevity and accuracy. Accordingly, there is a need for heating elements that are capable of producing a highly localized, predictable, and isothermal heating pattern.